Field of the Invention and Related Background Arts
The present invention relates to an image forming apparatus and method for forming an image by developing an electrostatic latent image formed on an image bearing member corresponding to an image to be recorded and recording it on a recording medium such as a plain paper. Furthermore, the present invention relates to a developing apparatus used for the image forming apparatus.
Heretofore, various types of electrophotographic developing apparatuses have been proposed and put into practical use. In general, they are classified into developing apparatuses using a one component developing system and developing apparatuses using a two component developing system. The one component system is a non-contact system in most cases, and as a representative example, there is a one component jumping developing system which uses a magnetic toner.
This developing system can provide a high quality image with a simple constitution. However, a clear color image cannot be provided because the toner contains a magnetic material. Further, in the case of the one component developing system using a nonmagnetic toner, although a color image can be provided, it is difficult to have the toner applied to a developing sleeve stably for a long period. Even when the non-magnetic toner is coated on the developing sleeve with an elastic blade, it is necessary to further improve its stability and durability.
On the other hand, in a two component developing method, development is carried out by conveying toner to a developing area using a magnetic carrier, and usually a developing process is carried out by bringing a developer into contact with a photosensitive drum. The developing process will be described with reference to FIG. 2. In FIG. 2, reference numeral 30 denotes a developing sleeve, reference numeral 35 a magnetic roller fixedly disposed inside the developing sleeve, reference numerals 31 and 32 agitating screws, reference numeral 33 a control blade disposed for forming the developer in a thin layer on the developing sleeve surface and reference numeral 34 a developer container.
The developing process for visualizing the above-described electrostatic latent image using a two component magnetic brush method, by using the above-described developing apparatus and a circulating system of the developer, will be described below. By rotation of the developing sleeve 30, developer drawn up at an N3 pole is controlled by the control blade 33 during the process of being conveyed from an S2 pole to an N1 pole and formed in a thin layer on the developing sleeve 30. When the developer formed in a thin layer is conveyed to a developing main pole S1, an ear-up is formed by a magnetic force. Using the developer formed in the ear-up shape, the above-described electrostatic latent image is developed, and, after that, the developer on the developing sleeve 30 is returned to the interior of the developing container 34 by a repulsive magnetic field of the N2 pole and the N3 pole. In the two component development method, as described above, magnetic poles generally having the same polarity are arranged side by side, and the unused developer, after the development process, is stripped off the developing sleeve so that no image hysteresis remains.
The developing sleeve is applied with a direct current bias and an alternating current bias from a power supply source (not shown). Generally, in the two component developing method, the application of an alternating current bias increases developing efficiency and enhances image quality.
As for latent image forming methods, there have been known such methods wherein an electrophotographic photosensitive member is scan-exposed by a laser beam, which is modulated in correspondence with the signal of an image to be recorded, and an electrostatic latent image is formed by distributing the latent image in a dot shape in correspondence with the image. Among those methods, however, the so-called pulse width modulation (PWM) method which modulates the width (that is, the duration) of a laser drive pulse current in correspondence with the shade of the image to be recorded can obtain a high recording density (that is, a high resolution) and also can obtain a high gradation.
In recent years, development has been made to attain miniaturization and longevity of the two component developing device. In order to achieve miniaturization from a development process side, it is necessary to miniaturize the container, the developing sleeve and the agitating conveyance screw. Making the space of a developer-reservoir portion smaller at a developer layer thickness control portion is also one of the effective methods. Execution of these steps of miniaturization and space reduction will severely reflect upon component accuracy and various latitudes and hence an elaboration of various ideas is necessitated.
In order to attain longevity, it is necessary to prevent deterioration of the toner and the carrier. For this purpose, there is required a constitution wherein the developer is not compressed. The location in which the developer is compressed is the developer layer thickness control portion and, in a usual constitution, the developer layer thickness control pole (the S2 pole in FIG. 2) is situated further in the upper stream of the rotational direction of the developing sleeve than the control blade and the developer attracted to a developer layer thickness control pole in this area is compressed between the sleeve and the container. To reduce the compression of the developer, it is most effective to reduce a force (Fr: a magnetic attraction force which works in the vertical direction relative to the developing sleeve) by which the developer layer thickness control pole attracts the developer to the developing sleeve.
As a method for that purpose, there can be cited such methods as minimizing a magnetization of the magnetic carrier inside the developer and constructing a magnet pattern in which a magnetic line of force from the developer layer thickness control pole is difficult to go around adjacent magnetic poles, but goes out as vertically as possible from the developing sleeve. Minimizing the magnetization of the carrier can be said to be one of various means for achieving high quality from a point that a force rubbing the toner image, which has been developed on the photosensitive member, becomes weaker.
As one of the latter methods, there has been proposed a developing method, which uses one of the repulsive magnetic poles of the magnet roller as a developer layer thickness control pole. When a repulsive magnetic field is formed with magnetic poles of the same polarity being side-by-side, the magnetic lines of force of each magnetic pole go out almost vertically to the developing sleeve. In this case, a rate of change in a magnetic flux density in the vertical direction relative to the developing sleeve is small. As a result, the force, which attracts the developer to the developing sleeve, becomes smaller and a condensation of the developer is reduced. It has been considered that, by implementing both of the above-described methods, still greater longevity can be attained.
Regarding a constitution to form the repulsive magnetic field, if the developer layer thickness control pole, which is one of the repulsive magnetic poles, is arranged downstream of the gravitational direction relative to the stripping off pole, which is another magnetic pole forming the repulsive magnetic field, it is much easier to draw up the developer to be coated on the developing sleeve and the constitution becomes simpler. As described above, such a constitution may possibly reduce the reservoir amount of the developer in the developer layer thickness control portion and may also lead to miniaturization of the developer container.
However, in the case where a constitution using one of the repulsive magnetic poles as the developer layer thickness control pole is adapted to a small size developing sleeve, a screw pitch-like density irregularity easily occurs at the rear end of a solid black image. This phenomenon occurs due to the fact that a mixing ratio of the developer (having an image hysteresis) with the toner density reduced, which moves to the developer layer thickness control pole after having been stripped off by the repulsive magnetic field, and the developer which is agitated and conveyed by the screw in the vicinity of the developing sleeve and supplied to the developer layer thickness control pole, changes by a rotational cycle of the screw at a longitudinal image area.
The phenomenon as described above is apt to occur when the developer surface (the topmost surface of the developer conveyed by the screw) is situated relatively downward in the gravitational direction inside the developer container and the agitating conveyer screw is installed in the vicinity of the developer layer thickness control pole. Further, this phenomenon is apt to occur when a size of magnetization of the magnetic carrier is small. This is because, in the case of the above-described constitution, the developer after a development process is difficult to strip off using a stripping pole and easy to move to the developer layer thickness control magnetic pole.
It is an object of the present invention to provide an image forming apparatus, an image forming method and a developing apparatus, wherein miniaturization and longevity of two component developer can be attained and a screw pitch-like density irregularity can be controlled.
It is an object of the present invention to provide an image forming apparatus, comprising: a non-magnetic rotatable cylinder having a diameter of 10 mm to 25 mm; a plurality of magnetic field generating means disposed inside the non-magnetic cylinder and fixed relative thereto; a developer container which houses a two component developer containing a magnetic carrier and a non-magnetic toner and has the non-magnetic cylinder rotatably mounted at an opening portion thereof; agitating conveyer means, disposed inside the developing container, which agitates the two component developer and conveys it to the non-magnetic cylinder; and an image bearing member arranged in opposition to the non-magnetic cylinder at intervals,
wherein the image forming apparatus conveys the two component developer by rotation of the non-magnetic cylinder and an electrostatic latent image on the image bearing member is magnetic-brush-developed,
the magnetic field generating means comprises: a developer layer thickness control pole for controlling a developer layer thickness on the non-magnetic cylinder of the developer supplied by the agitating conveyer means; and a stripping off pole having the same magnetic polarity as the developer layer thickness control pole and arranged adjacent to the thickness control pole in the upper side of the gravitational direction of the control pole and forming the magnetic field for stripping off the two component developer supplied for development from the non-magnetic cylinder,
the peak value of the strength of the magnetic field of the developer layer thickness control pole in the vertical direction relative to an outer peripheral surface of the non-magnetic cylinder is 0 tesla to 0.02 tesla in absolute value not less than the peak value of the strength of the magnetic field of the stripping off pole in the vertical direction relative to the outer peripheral surface, and
the agitating conveyer means is installed in such a manner that the topmost surface of the developer conveyed by the agitating conveyer means is situated between the peak position of the strength of the magnetic field of the developer layer thickness control pole in the vertical direction relative to the outer peripheral surface of the non-magnetic cylinder and the peak position of the strength of the magnetic field of the above described stripping off pole in such direction.
Further, it is an object of the present invention to provide an image forming method, comprising: providing a non-magnetic rotatable cylinder having a diameter of 10 mm to 25 mm; providing a plurality of magnetic field generating means disposed inside the non-magnetic cylinder and fixed relative thereto; providing a developing container which houses a two component developer containing a magnetic carrier and a non-magnetic toner, the non-magnetic cylinder being rotatably mounted at an opening portion thereof; providing agitating conveyer means, disposed inside the developer container, which agitates the two component developer and conveys it to the non-magnetic cylinder; and providing an image bearing member arranged in opposition to the non-magnetic cylinder at intervals, the two component developer being conveyed by rotation of the non-magnetic cylinder and an electrostatic latent image being magnetic-brush-developed on the image bearing member,
wherein providing the magnetic field generating means comprises: providing the developer layer thickness control pole for controlling the developer layer thickness on the non-magnetic cylinder of the developer supplied by the agitating conveyer means; and providing a stripping off pole having the same magnetic polarity as the developer layer thickness control pole and arranged adjacent to the thickness control pole in the upper side of the gravitational direction of the thickness control pole and forming the magnetic field for stripping off the two component developer supplied for development from the non-magnetic cylinder,
wherein the peak value of the strength of the magnetic field of the developer layer thickness control pole in the vertical direction relative to an outer peripheral surface of the non-magnetic cylinder is 0 tesla to 0.02 tesla in absolute value not less than the peak value of the strength of the magnetic field of the stripping off pole in the vertical direction relative to the outer peripheral surface, and
the agitating conveyer means is installed in such a manner that the topmost surface of the developer conveyed by the agitating conveyer means is situated between the peak position of the strength of the magnetic field of the developer layer thickness control pole in the vertical direction relative to the outer peripheral surface of the non-magnetic cylinder and the peak position of the strength of the magnetic field of the stripping off pole in such the direction.
Further, it is an object of the present invention to provide a developing apparatus, comprising: a non-magnetic rotatable cylinder having a diameter of 10 mm to 25 mm; a plurality of magnetic field generating means disposed inside the non-magnetic cylinder and fixed relative thereto; a developing container which houses a two component developer containing a magnetic carrier and a non-magnetic toner, the non-magnetic cylinder being rotatably mounted at an opening portion thereof; and agitating conveyer means, disposed inside the developer container, which agitates the two component developer and conveys it to the non-magnetic cylinder,
wherein the developing apparatus conveys the two component developer by rotation of the non-magnetic cylinder and an electrostatic latent image on the image bearing member of the image forming apparatus comprising the image bearing member arranged in opposition to the non-magnetic cylinder at intervals is magnetic-brush-developed,
the magnetic field generating means comprises: the developer layer thickness control pole for controlling the developer layer thickness on the non-magnetic cylinder of the developer supplied by the agitating conveyer means; and a stripping off pole having the same magnetic polarity as the developer layer thickness control pole and arranged adjacent to the thickness control pole in the upper side of the gravitational direction of the thickness control pole and forming the magnetic field for stripping off the two component developer supplied for development from the non-magnetic cylinder,
the peak value of the strength of the magnetic field of the developer layer thickness control pole in the vertical direction relative to an outer peripheral surface of the non-magnetic cylinder is 0 tesla to 0.02 tesla in absolute value not less than the peak value of the strength of the magnetic field of the stripping off pole in the vertical direction relative to the outer peripheral surface, and
the agitating conveyer means is installed in such a manner that the topmost surface of the developer conveyed by the agitating conveyer means is situated between the peak position of the strength of the magnetic field of the developer layer thickness control pole in the vertical direction relative to the outer peripheral surface of the non-magnetic cylinder and the peak position of the strength of the magnetic field of the stripping off pole in such direction.